Polymer blend compositions for use in engineering applications should exhibit a combination of physical properties such as heat resistance, good impact strength and good modulus. Additionally, the blend compositions should exhibit good melt flow properties which facilitate processing and molding of the blend compositions. Polycarbonates are popular blend components owing to their toughness and relatively high softening temperatures. However, owing to their relatively poor melt flow characteristics, polycarbonates are often blended with one or more additional polymers to improve their melt flow properties. Examples of such blend compositions include the Grabowski U.S. Pat. No. 3,130,177 which discloses blends of polycarbonates with polybutadiene, styrene, acrylonitrile graft polymers and the Grabowski U.S. Pat. No. 3,162,695 which discloses blends of polycarbonates with butadiene-styrene, methyl methacrylate, styrene graft copolymers. The Parsons U.S. Pat. No. 4,886,855 discloses blends of polycarbonate, ABS resin and styrene polymers which exhibit a good combination of physical properties. The Liebig et al U.S. Pat. No. 4,205,140 also discloses a thermoplastic molding composition comprising a blend of a polycarbonate, a diene rubber graft polymer such as ABS, and a styrene polymer. Similar blends of polycarbonate with styrenemaleic anhydride copolymer, ABS resin and styreneacrylonitrile random copolymer are disclosed in the Henton U.S. Pat. No. 4,218,544. The Grigo et al U.S. Pat. No. 4,472,554 discloses thermoplastic molding compositions comprising a blend of a polycarbonate, a graft polymer such as ABS and a polymeric acidifying agent. European Patent Applications Nos. 135,492 and 135,493 also disclose polymer blends of polycarbonates, ABS polymers and styrene polymers. These blends exhibit various physical properties depending on the type and ratio of components included therein.
It is also desirable that such polymer blend compositions exhibit flame retardant properties for use in various applications. While the addition of conventional flame retardant compounds provides a degree of flame retardancy to such polymer compositions, the addition of reasonable amounts of such conventional compounds does not provide sufficient flame retardancy to allow thin samples of the blend compositions (for example, sheets of a thickness of 0.060 inches) to pass such flammability tests as the UL 94 V or the UL 94 5V tests with VO ratings because of material dripping. The UL tests are standard test procedures of the Underwriters Laboratory to measure the flame retardant properties of a material. It has been known to include halogenated polymer components in such compositions in order to improve the flame retardant characteristics of the compositions. For example, ABS resins including one or more halogenated styrene components have been proposed for use in polycarbonate compositions. However, large amounts of the halogenated polymer component must often be included in order to achieve the desired flame retardant properties whereby one or more physical properties of the compositions are disadvantageously compromised.
Accordingly, a need exists for improved compositions of polycarbonate and ABS resin which exhibit satisfactory flame retardant properties without sacrificing the advantageous physical properties of such compositions.